High resistance material



iii

Patented Aug; 11, 1936 UNITED STATES;

. 2,050,357 PATENT OFFICE men RESISTANCE MATERIAL.

Leon McCulloch, Pittsburgh, -Pa., assignor to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., acorporation of Pennsylvania N 0 Drawing.

Appl ication December 28,1933, Serial No. 704,361 r 3 Claims. (01. 201- 15) 7 My invention relates to resistance materials having high values of electrical resistance and particularly to such materials derived by charring organic substances at a controlled temperatures,

and to a process of making the same.

It has been shown that organic substances may be blackened or charred by heat and still retain extremely high electrical resistance and insulating value. Charred material-so formed may, be-

' cause of this high resistance property, be utilized as an insulating pigment for electrical paints or varnishes requiring a high degree of electrical strength. Such a pigmented varnish is described and claimed in a copending application, Serial No.

perature is increased from 500 C. to 600 C.

Other organic materials may also be used, the

temperature at which the resistivity falls being difierent for diiferent materials, for example, the resistivity of cotton after prolonged charring falls rapidly with an increase in temperature from 300 C. to 500C.

In accordance with the present invention, it is proposed to utilize charred substances formed irom organic materials as electrical resistors, or

semi-conductors, having relati fh electrical resistivity, the desired value required being givenf;

to the material by baking it at the proper temperature during preparation of the material.

The resistivity retained by the charred material depends upon the maximum baking temperature and is practically independent of the length of time that the baking process is continued. Ihave heated pine wood for 10 days at a temperature of. between 350 C. and 450 C. and found that the resistance remained too high to be measured by available instruments. Such chars have been extracted with alkaline solutions and various organic solvents without disintegrating the chat or lowering its electrical resistance and given'ofl only Y very small amounts or substance in solution. The material being charred is contained in a nearly closed vessel practically excluding air, but

permitting vapors to escape from the vessel. If

air were permitted toenter the vessel,- organic substances such as wood would ignite and burn to ash. Organic substances other than. wood, such as asphalt, pitch, sugar, cotton and possibly bituminous coal might be'used. I have found,

- porcelain insulators.

In the case of char formed from .pine wood, the resistivity falls rapidly as the tem-' however, that pine wood is a desirable material on account of its low ash content.

Chars produced in accordance with my invention may be molded into electrical resistors under pressure and employed for various uses. Stable and. permanent electrical terminals or contacts for such resistors may be provided by molding.

low resistance conductors in intimate contact with the high resistance material.

One use of material, prepared in accordance with my invention, is as an aid in eliminating corona discharges about the surface of insulators.

This may be done by preparing a wood char from pine wood baked at 600 C. for a prolonged time,

the char being then ground and prepared as a putty for filling in the spaces about the tops of Another use for this material is as a paint to eliminate discharges about the ends of high voltage armature windings.

A satisfactory paint for this purpose may be made'as follows: Pine wood is heated at from 350 to 450 C. in a. partiallyclosed'vessel excluding air until smoke practically ceases. 'l'he'char thus formed is ground into a fine powder in a pebble mill or other suitable apparatus.

the temperature of which is automatically controlled for a particular temperature between 500 The- -powder is then baked in an electrical furnace,

C. and 600 C. according to the resistivity required. The time of baking employed is about 18 hours. Paints consisting of wood char as above described have been prepared in which 100 grams of the char was mixed with 100 cubic centimeters of air drying oil varnish and 40 cubic centimeters of petroleum naptha. The particularbinder used may be chosen according to the working characteristics of the paint required.

Paints prepared in the manner above described were applied to both sides of a three-quarter inch cotton tape in which the pigment was rubbed well into the fabric showed the following resistance per inch of tape according to the It willbe observed that thetemperature of' the furnace during the baking process must be closely controlled in order to produce a material having a desired predetermined resistance value.-

Such paints applied to coils and similar electrlcal apparatus by brushingprove very eifective in eliminating corona therefrom at very high temperature of' voltages. Alimitedamountoioilisusedinape0 access? plying such paints in order that the resistance til the resistivity of the n aterial reaches a con- I of the paint will be the resistance of the material itself instead or the contact resistance of the particles included therein; 7

I have also used shellac varnishes asa surfacing and binding material for the pigment with -very satisfactory results.

It will he understood that the description or particular embodiments of my invention is to be taken as illustrative of the principle and mode thereof and that modifications may be made within the spirit or the invention, without de parting from the scope oi the appended claims I claim as my invention: 1. The process of preparing material having a predetermined high value of electrical resistivity comprising heating pine wood at between 350 C. and 450 C. in an atmosphere substially free from oxygen until smoking nearly ceases, grinding the resulting char and baking at a pre=-' deteed temperature hetween'i t" C. and 806 (2., accor to the resistivity required, un-

'tivity required, until the resistivity of t value. 2. The process of preparing material having a predetermined value 01' electrical resistivitycomprising heating pine wood in an atmosphere substantially tree from-oxygen, at a temperature sumciently high to char it, until substantially free from volatile substances, grinding the resultin: char and baking at a predetermined temperature above 500 0., according to the resistivity required, until the resistivity of the material reaches a constant value.

3. The process of preparing material having a predetermined high value of electrical resistivity comprising heating wood in an atmosphere substantially free from oxygen at a temperature sufliciently high to char it, until substantially free from volatile substances, grinding the resulting char and baking at a predetermined tem perature above 500 0., according to the resisthe material reaches a constant value. LEON M. 

